Have "U" section outer race bearings been mentioned on this thread. I have seen them in some bearing supply catalogues and wondered, apart from the fact that they may be a bit narrow, whether one of these could replace the two races in parallel on your arm.
jamikl

I think rubber o rings would add friction to inhibit vertical play, along with preventing a mechanical path to ground for resonance control.

Colin

I think that there would be low friction, but even a little might be beneficial. As regards the resonance, (I assume you mean of the arm wands?) if you think this would be a problem, connecting the bearings using non-compliant/non-damping materials would allow the vibrations to continue. At least the rubber o-rings would add a bit of damping. And don't forget non-compliant mounting is two way, so what is stopping vibrations coming up through the arm base into the arm?

I like the idea of the design, and I have most of the bits already, I'm just going to have to try it.